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Understanding Current, Resistance, and Electromotive Force in Electrical Circuits

This chapter explores the fundamental concepts of current and resistance in electrical circuits. It defines current as the rate of flow of electric charge measured in amperes (A) and discusses the relationship between charge and time, Kirchhoff’s laws, and the nature of charge carriers. Additionally, the chapter covers current density, drift velocity, Ohm’s Law, and the difference between ohmic and non-ohmic materials. Energy and power are also examined, including practical examples like kilowatt-hours, to illustrate energy dissipation in electrical systems.

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Understanding Current, Resistance, and Electromotive Force in Electrical Circuits

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  1. Chapter 25 Current, Resistance, and Electromotive Force

  2. Current Current = Rate of flow of charge Unit: A (Ampere) 1 A = 1C/s Current flows from high potential to low potential

  3. Total Amount of Charge Suppose a wire carries current I. Find the total charge passing through the wire in time t.

  4. Conservation of charge(Kirchhoff’s junction rule) Arrows: Directions of POSITIVE charge flow We now know that current (in metals) is in fact caused by negatively charged electrons flowing in the opposite direction.

  5. Positive or negative charge carriers

  6. Current Density Current Density: Current per unit area Unit: A/m2 a b

  7. Direction Left or Right? Right Right Right

  8. Drift Velocity Closing time at the bar Drift velocity: vd

  9. Derivation of Drift Velocity

  10. Find the drift velocity Given n=8.49 × 1028 m-3 for copper, what is the drift velocity of electrons in a copper wire with radius r=900μm and I =17 mA?

  11. Resistance

  12. Ohm’s Law

  13. Resistor

  14. Resistivityρ andConductivity σ

  15. Resistivity

  16. Which has the higher resistance?

  17. Ohmic Material

  18. Nonohmic Material, Graph

  19. Power Power: Energy per unit time Unit: W (Watt) 1W=1J/s

  20. Example

  21. Kilowatt-hour and Joules Both measure energy 1kWh is the amount of energy dissipated by a 1000 Watts light bulb in one hour. Therefore: 1kWh = (1000J/s) (3600s)=3.6 × 106J

  22. Pumping Charges

  23. Kirchhoff’s Loop rule

  24. Ideal voltage source In this class, unless stated otherwise, we often assume voltage source is ideal, i.e., battery with no internal resistance (r =0).

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